Analyte sensor with insertion monitor, and methods

ABSTRACT

A sensor, and methods of making, for determining the concentration of an analyte, such as glucose or lactate, in a biological fluid such as blood or serum, using techniques such as coulometry, amperometry, and potentiometry. The sensor includes a working electrode and a counter electrode, and can include an insertion monitoring trace to determine correct positioning of the sensor in a connector.

This application is a continuation-in-part of U.S. Ser. No. 10/866,477,filed Jun. 12, 2004, which is a continuation of U.S. Ser. No.10/033,575, filed Dec. 28, 2001, issued as U.S. Pat. No. 6,749,740,which is a continuation of U.S. Ser. No. 09/434,026, filed Nov. 4, 1999,issued as U.S. Pat. No. 6,616,819, the entire disclosures of which areincorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to analytical sensors for the detection ofbioanalytes in a small volume sample, and methods of making and usingthe sensors.

BACKGROUND

Analytical sensors are useful in chemistry and medicine to determine thepresence and concentration of a biological analyte. Such sensors areneeded, for example, to monitor glucose in diabetic patients and lactateduring critical care events.

Currently available technology measures bioanalytes in relatively largesample volumes, e.g., generally requiring 3 microliters or more of bloodor other biological fluid. These fluid samples are obtained from apatient, for example, using a needle and syringe, or by lancing aportion of the skin such as the fingertip and “milking” the area toobtain a useful sample volume. These procedures are inconvenient for thepatient, and often painful, particularly when frequent samples arerequired. Less painful methods for obtaining a sample are known such aslancing the arm or thigh, which have a lower nerve ending density.However, lancing the body in the preferred regions typically producessubmicroliter samples of blood, because these regions are not heavilysupplied with near-surface capillary vessels.

It would therefore be desirable and very useful to develop a relativelypainless, easy to use blood analyte sensor, capable of performing anaccurate and sensitive analysis of the concentration of analytes in asmall volume of sample.

It would also be desirable to develop methods for manufacturing smallvolume electrochemical sensors capable of decreasing the errors thatarise from the size of the sensor and the sample.

SUMMARY OF THE DISCLOSURE

The sensors of the present invention provide a method for the detectionand quantification of an analyte. In general, the invention includes amethod and sensor for analysis of an analyte in a sample, e.g., a smallvolume sample, by, for example, coulometry, amperometry and/orpotentiometry. A sensor of the invention may utilize a non-leachable ordiffusible electron transfer agent and/or a redox mediator. The sensoralso includes a sample chamber to hold the sample in electrolyticcontact with the working electrode.

In one embodiment, the working electrode faces a counter electrode,forming a measurement zone within the sample chamber, between the twoelectrodes, that is sized to contain no more than about 1 μL of sample,e.g., no more than about 0.5 μL, e.g., no more than about 0.32 μL, e.g.,no more than about 0.25 μL, e.g., no more than about 0.1 μL of sample.

In one embodiment of the invention, a sensor, configured for insertioninto an electronic meter, is provided with a working electrode and acounter electrode, and a conductive insertion monitor which provideselectrical contact with the electronic meter if the sensor is properlyinserted into the meter. The conductive insertion monitor is configuredand arranged to close an electrical circuit when the sensor is properlyinserted into the electronic connector.

In another embodiment of the invention, a sensor is provided with aplurality of contacts, each contact having a contact pad, which is aregion for connection with an electronic meter. The plurality ofcontacts and contact pads are on a substrate having a length and awidth, and each contact pad has a contact pad width taken parallel tothe width of the substrate. The sum of the contact pad widths is greaterthan the width of the substrate. In one embodiment, six electricalconnections are made with six contact pads on the sensor but in a widththat is approximately the width of four contact pads. For example, aworking electrode, three counter electrodes (e.g., one counter electrodeand two indicator electrodes), and two insertion trace connections eachhave a contact pad; connection can be made to each of these six contactpads in the same width of the contact pads of the working electrode andthree counter electrodes.

The present invention also includes an electrical connector, forproviding electrical contact between a sensor and an electrical meter orother device. The electrical connector has a plurality of contactstructures, each which has a proximal contact end for electricalconnection to a sensor contact, and a distal end for electricalconnection to the electrical device. In one embodiment, a plurality offirst contact structures extend longitudinally parallel from the distalto the proximal end. Additionally, one or more second contractstructures extend longitudinally next to the first contact structures,from the distal end past the proximal end of the first contactstructures, and angle toward a longitudinal center line of theconnector. Contact to the sensor is then made via the proximal contactends.

In some embodiments, the electrical connector has at least two secondcontact structures extending longitudinally past the proximal end of thefirst contact structures and angling toward the longitudinal center lineof the connector. After the angled or bent portion, the proximal contactends of the second contact structures of one embodiment make electricalcontact with a single conductive surface of a sensor, such as aconductive insertion monitor. In another aspect, the first contactstructures can be configured and arranged to contact one or more workingand/or counter electrodes of a sensor, and the second contact structuresare configured and arranged to contact one or more conductive insertionmonitors.

The sensors of the present invention can be configured for side-fillingor tip-filling. In addition, in some embodiments, the sensor may be partof an integrated sample acquisition and analyte measurement device. Theintegrated sample acquisition and analyte measurement device can includethe sensor and a skin piercing member, so that the device can be used topierce the skin of a user to cause flow of a fluid sample, such asblood, that can then be collected by the sensor. In at least someembodiments, the fluid sample can be collected without moving theintegrated sample acquisition and analyte measurement device.

In one embodiment, the sensor is connected with an electrical device, toprovide a processor coupled to the sensor. The processor is configuredand arranged to determine, during electrolysis of a sample in the samplechamber, a series of current values. The processor determines a peakcurrent value from the series of current values. After the currentvalues decrease below a threshold fraction of the peak current values,slope values are determined from the current values and represent alinear function of the logarithm of current values over time. Theprocessor determines, from the slope values, an extrapolation slope.From the extrapolated slope and the measured current values, theprocessor determines an amount of charge needed to electrolyze thesample and, from that amount of charge, the concentration of the analytein the sample.

One method of forming a sensor, as described above, includes forming atleast one working electrode on a first substrate and forming at leastone counter or counter/reference electrode on a second substrate. Aspacer layer is disposed on either the first or second substrates. Thespacer layer defines a chamber into which a sample can be drawn and heldwhen the sensor is completed. A redox mediator and/or second electrontransfer agent can be disposed on the first or second substrate in aregion that will be exposed within the sample chamber when the sensor iscompleted. The first and second substrates are then brought together andspaced apart by the spacer layer with the sample chamber providingaccess to the at least one working electrode and the at least onecounter or counter/reference electrode. In some embodiments, the firstand second substrates are portions of a single sheet or continuous webof material. The invention includes particularly efficient and reliablemethods for the manufacture of these sensors.

One such efficient and reliable method includes providing an adhesivehaving first and second surfaces covered with first and second releaseliners and then making detailed cuts through the first release liner andthe adhesive but not through the second release liner. These cuts defineone or more sample chamber regions. A portion of the first release lineris removed to expose a portion of the first adhesive surface, whichleaves a remaining portion of the first release liner over the samplechamber regions. This exposed first adhesive surface is applied to afirst substrate having one or more conductive traces disposed thereon.The second release liner is removed together with the adhesive and thefirst release liner of the sample chamber regions in order to expose thesecond adhesive surface. The second adhesive surface is then applied toa second substrate having one or more conductive traces disposedthereon. This method forms a sensor having a sample chambercorresponding to one of the sample chamber regions.

These and various other features which characterize the invention arepointed out with particularity in the attached claims. For a betterunderstanding of the invention, its advantages, and objectives obtainedby its use, reference should be made to the drawings and to theaccompanying description, in which there is illustrated and describedpreferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings, wherein like reference numerals andletters indicate corresponding structure throughout the several views:

FIG. 1 is a schematic view of a first embodiment of a sensor strip inaccordance with the present invention;

FIG. 2A is an exploded view of the sensor strip shown in FIG. 1, thelayers illustrated individually with the electrodes in a firstconfiguration;

FIG. 2B is a top view of the sensor strip shown in FIGS. 1 and 2A;

FIG. 3A is a schematic view of a second embodiment of a sensor strip inaccordance with the present invention, the layer illustratedindividually with the electrodes in a second configuration;

FIG. 3B is a top view of the sensor strip shown in FIG. 3A;

FIG. 4 is a top view of the first substrate of the sensor strip of FIGS.3A and 3B;

FIG. 5A is a top view of a first example configuration for a suitableinsertion monitor in accordance with the present invention;

FIG. 5B is a top view of a second example configuration for a suitableinsertion monitor in accordance with the present invention;

FIG. 5C is a top view of a third example configuration for a suitableinsertion monitor in accordance with the present invention;

FIG. 5D is a top view of a fourth example configuration for a suitableinsertion monitor in accordance with the present invention;

FIG. 6A illustrates a top view of one embodiment of a sheet of sensorcomponents, according to the invention;

FIG. 6B illustrates a top view of another embodiment of a sheet ofsensor components, according to the invention;

FIG. 7A is a top perspective view of a sensor strip positioned forinsertion within an electrical connector device in accordance with thepresent invention;

FIG. 7B is an exploded view of the electrical connector device of FIG.7A;

FIG. 8A is a top perspective view of a sensor strip fully positionedwithin the electrical connector device of FIG. 7A;

FIG. 8B is an exploded view of the electrical connector device of FIG.8A;

FIG. 9A is a bottom perspective view of the electrical connector deviceof FIGS. 7A and 7B; and

FIG. 9B is a bottom perspective view of the electrical connector deviceof FIGS. 8A and 8B.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As used herein, the following definitions define the stated term:

“Amperometry” includes steady-state amperometry, chronoamperometry, andCottrell-type measurements.

A “biological fluid” is any body fluid in which the analyte can bemeasured, for example, blood (which includes whole blood and itscell-free components, such as, plasma and serum), interstitial fluid,dermal fluid, sweat, tears, urine and saliva.

“Coulometry” is the determination of charge passed or projected to passduring complete or nearly complete electrolysis of the analyte, eitherdirectly on the electrode or through one or more electron transferagents. The charge is determined by measurement of charge passed duringpartial or nearly complete electrolysis of the analyte or, more often,by multiple measurements during the electrolysis of a decaying currentand elapsed time. The decaying current results from the decline in theconcentration of the electrolyzed species caused by the electrolysis.

A “counter electrode” refers to one or more electrodes paired with theworking electrode, through which passes an electrochemical current equalin magnitude and opposite in sign to the current passed through theworking electrode. The term “counter electrode” is meant to includecounter electrodes which also function as reference electrodes (i.e. acounter/reference electrode) unless the description provides that a“counter electrode” excludes a reference or counter/reference electrode.

An “electrochemical sensor” is a device configured to detect thepresence of and/or measure the concentration of an analyte viaelectrochemical oxidation and reduction reactions. These reactions aretransduced to an electrical signal that can be correlated to an amountor concentration of analyte.

“Electrolysis” is the electrooxidation or electroreduction of a compoundeither directly at an electrode or via one or more electron transferagents (e.g., redox mediators and/or enzymes).

The term “facing electrodes” refers to a configuration of the workingand counter electrodes in which the working surface of the workingelectrode is disposed in approximate opposition to a surface of thecounter electrode. In at least some instances, the distance between theworking and counter electrodes is less than the width of the workingsurface of the working electrode.

An “indicator electrode” or “fill indicator electrode” is an electrodethat detects partial or complete filling of a sample chamber and/ormeasurement zone with sample.

A “layer” is one or more layers.

The “measurement zone” is defined herein as a region of the samplechamber sized to contain only that portion of the sample that is to beinterrogated during an analyte assay.

A “non-diffusible,” “non-leachable,” or “non-releasable” compound is acompound which does not substantially diffuse away from the workingsurface of the working electrode for the duration of the analyte assay.

A “redox mediator” is an electron transfer agent for carrying electronsbetween the analyte and the working electrode, either directly orthrough another electron transfer agent.

A “reference electrode” includes a reference electrode that alsofunctions as a counter electrode (i.e., a counter/reference electrode)unless the description provides that a “reference electrode” excludes acounter/reference electrode.

A “working electrode” is an electrode at which analyte iselectrooxidized or electroreduced with or without the agency of a redoxmediator.

Referring to the Drawings in general and FIGS. 1 and 2A in particular, afirst embodiment of a sensor strip 10 is schematically illustrated.Sensor strip 10 has a first substrate 12, a second substrate 14, and aspacer 15 positioned therebetween. Sensor strip 10 includes at least oneworking electrode 22 and at least one counter electrode 24. Sensor strip10 also includes insertion monitor 30.

Sensor Strips

Referring to FIGS. 1, 2A and 2B in particular, sensor strip 10 has firstsubstrate 12, second substrate 14, and spacer 15 positionedtherebetween. Sensor strip 10 includes working electrode 22, counterelectrode 24 and insertion monitor 30. Sensor strip 10 is a layeredconstruction, in certain embodiments having a generally rectangularshape, i.e., its length is longer than its width, although other shapesare possible as well. Sensor strip 10′ of FIGS. 3A, 3B and 4 also hasfirst substrate 12, second substrate 14, spacer 15, working electrode22, counter electrode 24 and insertion monitor 30.

The dimensions of a sensor may vary. In certain embodiments, the overalllength of sensor strip 10, 10′ may be no less than about 20 mm and nogreater than about 50 mm. For example, the length may be between about30 and 45 mm; e.g., about 30 to 40 mm. It is understood, however thatshorter and longer sensor strips 10, 10′ could be made. In certainembodiments, the overall width of sensor strip 10, 10′ may be no lessthan about 3 mm and no greater than about 15 mm. For example, the widthmay be between about 4 and 10 mm, about 5 to 8 mm, or about 5 to 6 mm.In one particular example, sensor strip 10, 10′ has a length of about 32mm and a width of about 6 mm. In another particular example, sensorstrip 10, 10′ has a length of about 40 mm and a width of about 5 mm. Inyet another particular example, sensor strip 10, 10′ has a length ofabout 34 mm and a width of about 5 mm.

Substrates

As provided above, sensor strip 10, 10′ has first and second substrates12, 14, non-conducting, inert substrates which form the overall shapeand size of sensor strip 10, 10′. Substrates 12, 14 may be substantiallyrigid or substantially flexible. In certain embodiments, substrates 12,14 are flexible or deformable. Examples of suitable materials forsubstrates 12, 14 include, but are not limited, to polyester,polyethylene, polycarbonate, polypropylene, nylon, and other “plastics”or polymers. In certain embodiments the substrate material is “Melinex”polyester. Other non-conducting materials may also be used.

Spacer Layer

As indicated above, positioned between substrate 12 and substrate 14 canbe spacer 15 to separate first substrate 12 from second substrate 14.Spacer 15 is an inert non-conducting substrate, typically at least asflexible and deformable (or as rigid) as substrates 12, 14. In certainembodiments, spacer 15 is an adhesive layer or double-sided adhesivetape or film. Any adhesive selected for spacer 15 should be selected tonot diffuse or release material which may interfere with accurateanalyte measurement.

In certain embodiments, the thickness of spacer 15 may be at least about0.01 mm (10 μm) and no greater than about 1 mm or about 0.5 mm. Forexample, the thickness may be between about 0.02 mm (20 μm) and about0.2 mm (200 μm). In one certain embodiment, the thickness is about 0.05mm (50 μm), and about 0.1 mm (100 μm) in another embodiment.

Sample Chamber

The sensor includes a sample chamber for receiving a volume of sample tobe analyzed; in the embodiment illustrated, particularly in FIG. 1,sensor strip 10, 10′ includes sample chamber 20 having an inlet 21 foraccess to sample chamber 20. In the embodiments illustrated, sensorstrips 10, 10′ are side-fill sensor strips, having inlet 21 present on aside edge of strips 10, 10′. Tip-fill sensors can also be configured inaccordance with this invention.

Sample chamber 20 is configured so that when a sample is provided inchamber 20, the sample is in electrolytic contact with both the workingelectrode and the counter electrode, which allows electrical current toflow between the electrodes to effect the electrolysis (electrooxidationor electroreduction) of the analyte.

Sample chamber 20 is defined by substrate 12, substrate 14 and spacer15; in many embodiments, sample chamber 20 exists between substrate 12and substrate 14 where spacer 15 is not present. Typically, a portion ofspacer 15 is removed to provide an area between substrates 12, 14without spacer 15; this volume of removed spacer is sample chamber 20.For embodiments that include spacer 15 between substrates 12, 14, thethickness of sample chamber 20 is generally the thickness of spacer 15.

Sample chamber 20 has a volume sufficient to receive a sample ofbiological fluid therein. In some embodiments, such as when sensor strip10, 10′ is a small volume sensor, sample chamber 20 has a volume that ispreferably no more than about 1 μL, for example no more than about 0.5μL, and also for example, no more than about 0.25 μL. A volume of nomore than about 0.1 μL is also suitable for sample chamber 20, as arevolumes of no more than about 0.05 μL and about 0.03 μL.

A measurement zone is contained within sample chamber 20 and is theregion of the sample chamber that contains only that portion of thesample that is interrogated during the analyte assay. In some designs,the measurement zone has a volume that is approximately equal to thevolume of sample chamber 20. In some embodiments the measurement zoneincludes 80% of the sample chamber, 90% in other embodiments, and about100% in yet other embodiments.

As provided above, the thickness of sample chamber 20 correspondstypically to the thickness of spacer 15. Particularly for facingelectrode configurations, this thickness is small to promote rapidelectrolysis of the analyte, as more of the sample will be in contactwith the electrode surface for a given sample volume. In addition, athin sample chamber 20 helps to reduce errors from diffusion of analyteinto the measurement zone from other portions of the sample chamberduring the analyte assay, because diffusion time is long relative to themeasurement time, which may be about 5 seconds or less.

Electrodes

As provided above, the sensor includes a working electrode and at leastone counter electrode. The counter electrode may be a counter/referenceelectrode. If multiple counter electrodes are present, one of thecounter electrodes will be a counter electrode and one or more may bereference electrodes. Referring to FIGS. 2A and 2B and FIGS. 3A, 3B and4, two examples of suitable electrode configurations are illustrated.

Working Electrode

At least one working electrode is positioned on one of first substrate12 and second substrate 14. In all of FIGS. 2A though 4, workingelectrode 22 is illustrated on substrate 12. Working electrode 22extends from the sample chamber 20 to the other end of the sensor 10 asan electrode extension called a “trace”. The trace provides a contactpad 23 for providing electrical connection to a meter or other device toallow for data and measurement collection, as will be described later.Contact pad 23 can be positioned on a tab 26 that extends from thesubstrate on which working electrode 22 is positioned, such as substrate12. In one embodiment, a tab has more than one contact pad positionedthereon. In a second embodiment, a single contact pad is used to providea connection to one or more electrodes; that is, multiple electrodes arecoupled together and are connected via one contact pad.

Working electrode 22 can be a layer of conductive material such as gold,carbon, platinum, ruthenium dioxide, palladium, or other non-corroding,conducting material. Working electrode 22 can be a combination of two ormore conductive materials. An example of a suitable conductive epoxy isECCOCOAT CT5079-3 Carbon-Filled Conductive Epoxy Coating (available fromW.R. Grace Company, Woburn, Mass.). The material of working electrode 22typically has relatively low electrical resistance and is typicallyelectrochemically inert over the potential range of the sensor duringoperation.

Working electrode 22 may be applied on substrate 12 by any of variousmethods, including by being deposited, such as by vapor deposition orvacuum deposition or otherwise sputtered, printed on a flat surface orin an embossed or otherwise recessed surface, transferred from aseparate carrier or liner, etched, or molded. Suitable methods ofprinting include screen-printing, piezoelectric printing, ink jetprinting, laser printing, photolithography, and painting.

As provided above, at least a portion of working electrode 22 isprovided in sample chamber 20 for the analysis of analyte, inconjunction with the counter electrode.

Counter Electrode

The sensor includes at least one counter electrode positioned within thesample chamber. In FIGS. 2A and 2B, counter electrode 24 is illustratedon substrate 14. In FIGS. 3A, 3B and 4, a counter electrode 24 ispresent on substrate 12. Counter electrode 24 extends from the samplechamber 20 to the other end of the sensor 10 as an electrode extensioncalled a “trace”. The trace provides a contact pad 25 for providingelectrical connection to a meter or other device to allow for data andmeasurement collection, as will be described later. Contact pad 25 canbe positioned on a tab 27 that extends from the substrate on whichcounter electrode 24 is positioned, such as substrate 12 or 14. In oneembodiment, a tab has more than one contact pad positioned thereon. In asecond embodiment, a single contact pad is used to provide a connectionto one or more electrodes; that is, multiple electrodes are coupledtogether and are connected via one contact pad.

Counter electrode 24 may be constructed in a manner similar to workingelectrode 22. Suitable materials for the counter/reference or referenceelectrode include Ag/AgCl or Ag/AgBr on a non-conducting base materialor silver chloride on a silver metal base. The same materials andmethods may be used for counter electrode 24 as are available forworking electrode 22, although different materials and methods may alsobe used. Counter electrode 24 can include a mix of multiple conductingmaterials, such as Ag/AgCl and carbon.

Electrode Configurations

Working electrode 22 and counter electrode 24 may be disposed oppositeto and facing each other to form facing electrodes. See for example,FIG. 2A, which has working electrode 22 on substrate 12 and counterelectrode 24 on substrate 14, forming facing electrodes. In thisconfiguration, the sample chamber is typically present between the twoelectrodes 22, 24. For this facing electrode configuration, electrodes22, 24 may be separated by a distance of no more than about 0.2 mm(e.g., at least one portion of the working electrode is separated fromone portion of the counter electrode by no more than about 200 μm),e.g., no more than about 100 μm, e.g., no more than about 50 μm.

Working electrode 22 and counter electrode 24 can alternately bedisposed generally planar to one another, such as on the same substrate,to form co-planar or planar electrodes. Referring to FIGS. 3A and 4,both working electrode 22 and counter electrode 24 occupy a portion ofthe surface of substrate 12, thus forming co-planar electrodes.

Sensing Chemistry

In addition to working electrode 22, sensing chemistry material(s) arepreferably provided in sample chamber 20 for the analysis of theanalyte. Sensing chemistry material facilitates the transfer ofelectrons between working electrode 22 and the analyte in the sample.Any sensing chemistry may be used in sensor strip 10, 10′; the sensingchemistry may include one or more materials.

The sensing chemistry can be diffusible or leachable, or non-diffusibleor non-leachable. For purposes of discussion herein, the term“diffusible” will be used to represent “diffusible or leachable” and theterm “non-diffusible” will be used to represent “non-diffusible ornon-leachable” and variations thereof. Placement of sensing chemistrycomponents may depend on whether they are diffusible or not. Forexample, both non-diffusible and/or diffusible component(s) may form asensing layer on working electrode 22. Alternatively, one or morediffusible components may be present on any surface in sample chamber 20prior to the introduction of the sample to be analyzed. As anotherexample, one or more diffusible component(s) may be placed in the sampleprior to introduction of the sample into sample chamber 20.

Electron Transfer Agent

The sensing chemistry generally includes an electron transfer agent thatfacilitates the transfer of electrons to or from the analyte. Theelectron transfer agent may be diffusible or non-diffusible, and may bepresent on working electrode 22 as a layer. One example of a suitableelectron transfer agent is an enzyme which catalyzes a reaction of theanalyte. For example, a glucose oxidase or glucose dehydrogenase, suchas pyrroloquinoline quinone glucose dehydrogenase (PQQ), is used whenthe analyte is glucose. Other enzymes can be used for other analytes.

The electron transfer agent, whether it is diffusible or not,facilitates a current between working electrode 22 and the analyte andenables the electrochemical analysis of molecules. The agent facilitatesthe transfer electrons between the electrode and the analyte.

Redox Mediator

This sensing chemistry may, additionally to or alternatively to theelectron transfer agent, include a redox mediator. Certain embodimentsuse a redox mediator that is a transition metal compound or complex.Examples of suitable transition metal compounds or complexes includeosmium, ruthenium, iron, and cobalt compounds or complexes. In thesecomplexes, the transition metal is coordinatively bound to one or moreligands, which are typically mono-, di-, tri-, or tetradentate. Theredox mediator can be a polymeric redox mediator, or, a redox polymer(i.e., a polymer having one or more redox species). Examples of suitableredox mediators and redox polymer are disclosed in U.S. Pat. No.6,338,790, for example, and in U.S. Pat. Nos. 6,605,200 and 6,605,201.

If the redox mediator is non-diffusible, then the redox mediator may bedisposed on working electrode 22 as a layer. In an embodiment having aredox mediator and an electron transfer agent, if the redox mediator andelectron transfer agent are both non-leachable, then both components aredisposed on working electrode 22 as individual layers, or combined andapplied as a single layer.

The redox mediator, whether it is diffusible or not, mediates a currentbetween working electrode 22 and the analyte and enables theelectrochemical analysis of molecules which may not be suited for directelectrochemical reaction on an electrode. The mediator functions as anagent to transfer electrons between the electrode and the analyte.

Sorbent Material

Sample chamber 20 can be empty before the sample is placed in thechamber, or, in some embodiments, the sample chamber can include asorbent material to sorb and hold a fluid sample during the measurementprocess. The sorbent material facilitates the uptake of small volumesamples by a wicking action which can complement or, e.g., replace anycapillary action of the sample chamber. Suitable sorbent materialsinclude polyester, nylon, cellulose, and cellulose derivatives such asnitrocellulose. In addition to or alternatively, a portion or theentirety of the wall of the sample chamber may be coated by asurfactant, which is intended to lower the surface tension of the fluidsample and improve fluid flow within the sample chamber.

Methods other than the wicking action of a sorbent can be used totransport the sample into the sample chamber or measurement zone.Examples of such methods for transport include the application ofpressure on a sample to push it into the sample chamber, the creation ofa vacuum by a pump or other vacuum-producing method in the samplechamber to pull the sample into the chamber, capillary action due tointerfacial tension of the sample with the walls of a thin samplechamber, as well as the wicking action of a sorbent material.

Fill Indicator Electrode

In some instances, it is desirable to be able to determine when thesample chamber is filled. Sensor strip 10, 10′ can be indicated asfilled, or substantially filled, by observing a signal between anindicator electrode and one or both of working electrode 22 or counterelectrode 24 as sample chamber 20 fills with fluid. When fluid reachesthe indicator electrode, the signal from that electrode will change.Suitable signals for observing include, for example, voltage, current,resistance, impedance, or capacitance between the indicator electrodeand, for example, working electrode 22. Alternatively, the sensor can beobserved after filling to determine if a value of the signal (e.g.,voltage, current, resistance, impedance, or capacitance) has beenreached indicating that the sample chamber is filled.

Typically, the indicator electrode is further downstream from a sampleinlet, such as inlet 21, than working electrode 22 and counter electrode24.

For side-fill sensors, an indicator electrode can be present on eachside of the counter electrode. This permits the user to fill the samplechamber from either the left or right side with an indicator electrodedisposed further upstream. This three-electrode configuration is notnecessary. Side-fill sensors can also have a single indicator electrodeand may include some indication as to which side should be placed incontact with the sample fluid.

The indicator electrode can also be used to improve the precision of theanalyte measurements. The indicator electrode may operate as a workingelectrode or as a counter electrode or counter/reference electrode.Measurements from the indicator electrode/working electrode can becombined (for example, added or averaged) with those from the firstcounter/reference electrode/working electrode to obtain more accuratemeasurements.

The sensor or equipment that the sensor connected is with (e.g., ameter) can include a sign (e.g., a visual sign or auditory signal) thatis activated in response to the indicator electrode to alert the userthat the measurement zone has been filled. The sensor or equipment canbe configured to initiate a reading when the indicator electrodeindicates that the measurement zone has been filled with or withoutalerting the user. The reading can be initiated, for example, byapplying a potential between the working electrode and the counterelectrode and beginning to monitor the signals generated at the workingelectrode.

Insertion Monitor

In accordance with this invention, the sensor includes an indicator tonotify when proper insertion of sensor strip 10, 10′ into receivingequipment, such as a meter, has occurred. As seen in FIGS. 1, 2A, 2B, 3Aand 3B, sensor strips 10, 10′ include insertion monitor 30 on anexterior surface of one of substrates 12, 14.

Insertion monitor 30 is used to encode information regarding sensorstrip 10, 10′. The encoded information can be, for example, calibrationinformation for that manufacturing lot or for that specific strip. Suchcalibration information or code may relate to, e.g., the sensitivity ofthe strip or to the y-intercept and/or slope of its calibration curve.The calibration code is used by the meter or other equipment to whichsensor strip 10, 10′ is connected to provide an accurate analytereading. For example, based on the calibration code, the meter uses oneof several programs stored within the meter.

In some embodiments, a value indicative of the calibration code ismanually entered into the meter or other equipment, for example, by theuser. In other embodiments, the calibration code is directly read by themeter or other equipment, thus not requiring input or other interactionby the user.

In one embodiment, illustrated, for example in FIG. 5A, insertionmonitor 30 is a stripe 130 extending across an exterior surface ofsensor 10, 10′, for example, from side edge to side edge, with onecontact pad for connection to a meter. It is understood that inalternate embodiments stripe 130 need not extend to both side edges. Inanother embodiment, the insertion monitor comprises two or more contactpads for connection to a meter. The two or more contact pads areelectrically connected to each other by a material, such as a conductiveink.

The calibration code can be designed into insertion monitor 30, forexample, either by the resistance or other electrical characteristic ofinsertion monitor 30, by the placement or position of insertion monitor30, or by the shape or configuration of insertion monitor 30.

Insertion monitor 30 may alternately or additionally carry otherinformation regarding the sensor strip 10, 10′. This other informationthat could be encoded into insertion monitor 30 include the test timeneeded for accurate analyte concentration analysis, expiration date ofthe sensor strip 10, 10′, various correction factors, such as forenvironmental temperature and/or pressure, selection of the analyte tobe analyzed (e.g., glucose, ketone, lactate), and the like.

The resistance of insertion monitor 30, such as that of single stripe130 or area or of a conductive path between the two or more contactpads, is related to the encoded information. As an example of discretecalibration values, resistance values in a given range can correspond toone calibration setting, and resistance values in a different range cancorrespond to a different calibration setting. Thus, when a meter orother equipment receives a sensor strip, indicator monitor 30 willnotify the meter or equipment which assay calculation to use.

In addition to varying the resistance of indicator monitor 30 by varyingthe conductive or semi-conductive material used, the resistance ofindicator monitor 30 can be varied by cutting or scoring some or all ofthe conductive pathways so that they do not carry charge. The resistancecan additionally or alternately be controlled by the width or length ofthe conductive path. An example of a material suitable for indicatormonitor 30 is a combination of carbon and silver; the resistance of thismixture will vary, based on the ratio of the two materials.

The placement or position of insertion monitor 30 can additionally oralternately be related to the encoded calibration information. Forexample, the calibration code can be directly related to the location ofindicator monitor 30. For example, the position of indicator monitor 30can be varied so that is makes electrical contact with different contactstructures. (Contact structures are described below in “SensorConnection to Electrical Device”). Depending on the contact structuresengaged, the meter will recognize the calibration code and thus knowwhat parameter to use to calculate an accurate analyte level.

The shape and/or configuration of insertion monitor 30 can additionallyor alternatively be related to the encoded calibration code. Forexample, the calibration code can be directed related to which and/orthe number of contact structures that make electrical contact withindicator monitor 30. For example, a pattern of discrete and unconnectedindicator monitors can be present on the sensor; the calibration codewill be directly related to the arrangement of those monitors. Thepattern could be parallel lines, orderly arranged dots or squares, orthe like.

While it is preferred to provide this encoded information on theinsertion monitor, it should be recognized that the insertion monitorfunction and the encoding of information can also be implementedseparately using separate conductive traces on the strip.

Conductive insertion monitor 30 is positioned on the non-conductive basesubstrate and has a contact pad for electrical contact with a connector.Insertion monitor 30 is configured and arranged to close an electricalcircuit when sensor 10, 10′ is properly inserted into the connector.

Insertion monitor 30 may have any suitable configuration, including butnot limited to, a stripe extending across sensor strip 10, 10′ from aside edge to a side edge, such as stripe 130, a stripe extending acrossthe sensor strip, although not the entire width, and an array ofunconnected dots, strips, or other areas. Other suitable configurationsfor insertion monitor 30 are illustrated in FIGS. 5B, 5C and 5D. FIG. 5Billustrates insertion monitor 30 as bi-regional monitor 230, having afirst stripe 230A and a second stripe 230B, both of which extend fromside edge to side edge, although it is understood that one or both ofstrips 230A, 230B may not extend completely to a side edge. FIGS. 5C and5D illustrate insertion monitors that have a long, tortuous path, whichextends longitudinally toward an end of the sensor, rather thanextending merely side-to-side. Insertion monitor 330 of FIG. 5C has astripe 330A and an elongate stripe 330B. Insertion monitor 430 of FIG.5D has a single conductive strip 430, which provides an elongate path.

Sensor Connection to Electrical Device

Referring to FIGS. 7A, 7B, 8A, 8B, 9A and 9B, a sensor strip 100 isillustrated readied for insertion into a connector 500. Sensor strip 100is similar to sensor strips 10, 10′. Sensor strip 100 includes insertionmonitor 30 on an outer surface of one of the substrates forming strip100. Sensor strip 100 includes, although not illustrated, one workingelectrode and three counter electrodes. The working electrode includes acontact pad positioned on tab 123 (see FIGS. 7A and 9A). Each of thethree counter electrodes includes a contact pad positioned on tab 124,125, 126, respectively (see FIG. 9A).

Sensor strip 100 is configured to couple to a meter or other electricaldevice by electrical connector 500 which is configured to couple withand contact the end of sensor 100 at contact pads 123, 124, 125, 126.The sensor meter typically includes a potentiostat or other component toprovide a potential and/or current for the electrodes of the sensor. Thesensor reader also typically includes a processor (e.g., amicroprocessor or hardware) for determining analyte concentration fromthe sensor signals. The sensor meter also includes a display or a portfor coupling a display to the sensor. The display displays the sensorsignals and/or results determined from the sensor signals including, forexample, analyte concentration, rate of change of analyte concentration,and/or the exceeding of a threshold analyte concentration (indicating,for example, hypo- or hyperglycemia).

One example of a suitable connector is shown in FIGS. 7A and 7B, 8A and8B, and 9A and 9B. Connector 500 (which is used to connect a sensor to ameter or other electrical device) is generally a two part structure,having top portion 510 and bottom portion 520 (see FIG. 7B). Positionedbetween and secured by top portion 510 and bottom portion 520 arevarious contact leads that provide electrical connection between sensor100 and a meter. Bottom portion includes leads 51, 52 and 223, 224, 225,226, as will be described below.

Leads 223, 224, 225, 226, have proximal ends to physically contact pads123, 124, 125, 126, respectively, and to connect to any attached meter.Each pad 123, 124, 125, 126 has its respective lead 223, 224, 225, 226.The end of sensor 100 having the contact pads can be slid into or matedwith connector 500 by placing sensor 100 into slide area 530, whichprovides a support for and retains sensor 100. It is typically importantthat the contact structures of the connector 500 make electrical contactwith the correct pads of the sensor so that the working electrode andcounter electrode(s) are correctly coupled to the meter.

Connector 500 includes leads or contact structures 51, 52 for connectionto insertion monitor 30. Insertion monitor 30 is configured and arrangedto close an electrical circuit between contact structures 51 and 52 whenthe sensor is properly inserted into the connector. Proper insertioninto connector 500 means that the sensor strip 100 is inserted rightside up, that the correct end of strip 100 is inserted into connector500, and that sensor strip 100 is inserted far enough into connector 500that reliable electrical connections are made between the electrodecontact pads 123, 124, 125, 126 and the corresponding contacts leads223, 224, 225, 226. Preferably, no closed circuit is made unless allelectrode pads have properly contacted the contact structures ofconnector 500. The insertion monitor may have shapes other than a stripeacross the width of the sensor; for example, other designs include anindividual dot, a grid pattern, or may include stylistic features, suchas words or letters.

Because this insertion monitor 30 is not at the end with the contactregions for the electrodes, the insertion monitor 30 does not requireadditional width space on the sensor. The width of the contact pads 123,124, 125, 126 is defined as the width on which a lead could be placedthat would result in an electrical connection; typically, the contactwidth is the width of the exposed contact area. In one embodiment, sixcontact lead structures on the connector (e.g., 52, 223, 224, 225, 226,51) can contact sensor 100 in the same width as the four contact pads(e.g., 123, 124, 125, 126). This concept of having contact points on thesensor that occupy more width than the width of the sensor may be usedfor any number of contact points; this may be used with or without aninsertion monitor 30.

As a particular example, four leads 223, 224, 225, 226 make contact withcontact pads 123, 124, 125, 126. If each lead and/or contact pad is onemillimeter wide, a sensor of at least 4 mm wide is needed to makecontact. Additional leads, such as those for insertion monitor 30 (i.e.,contact leads 51, 52), can make contact by having leads 51, 52 extendalong the side of leads 223, 226 and then angle in toward the center ofstrip 100 after the point where leads 223, 224, 225, 226 contact strip100. The insertion monitor leads 51, 52 cross side edges of sensor 100to make contact with the sensor, thus not requiring additional sensorwidth.

The contact structures are generally parallel and non-overlapping. Thelead structures 223, 224, 225, 226 terminate in close proximity to theproximal end of sensor strip 100 (e.g., on contact pads 123, 124, 125,126), but lead structures 51, 52 continue longitudinally past theproximal end of lead structures 223, 224, 225, 226 farther toward thedistal end of sensor strip 100. Once past the proximal end and past leadstructures 223, 224, 225, 226, lead structures 51, 52 angle in towardthe center of the sensor strip.

In an optional embodiment to ensure proper insertion of a sensor into ameter, the meter may include a raised area or bump that prevents orhinders the insertion of the sensor in an improper direction. Objectsother than a raised area can also be used to guide the user in correctintroduction of the sensor into the meter.

General Method for Manufacturing Sensors

Referring now to FIGS. 6A and 6B, one example of a method for makingsensors having two substrates with electrodes thereon is described withrespect to the sensor arrangement displayed in FIG. 2A, although thismethod can be used to make a variety of other sensor arrangements,including those described before. When the three layers of FIG. 2A areassembled, a sensor similar to sensor 10 is formed.

In FIGS. 6A and 6B, a substrate 1000, such as a plastic substrate, ismoving in the direction indicated by the arrow. Substrate 1000 can be anindividual sheet or a continuous roll on a web. Multiple sensors can beformed on substrate 1000 as sections 1022 that have working electrodes22 (FIG. 2A) thereon and sections 1024 that have counter electrodes 24(FIG. 2A) thereon and other electrodes, such as reference electrodesand/or fill indicator electrodes. These working, counter and optionalelectrodes are electrically connected to their corresponding traces andcontact pads. Typically, working electrode sections 1022 are produced onone half of substrate 1000 and counter electrode sections 1024 areproduce on the other half of substrate 1000. In some embodiments,substrate 1000 can be scored and folded to bring the sections 1022, 1024together to form the sensor. In some embodiments, as illustrated in FIG.6A, the individual working electrode sections 1022 can be formed next toor adjacent each other on substrate 1000, to reduce waste material.Similarly, individual counter electrode sections 1024 can be formed nextto or adjacent each other. In other embodiments, the individual workingelectrode sections 1022 (and, similarly, the counter electrode sections1024) can be spaced apart, as illustrated in FIG. 6B. The remainder ofthe process is described for the manufacture of multiple sensors, butcan be readily modified to form individual sensors.

Carbon or other electrode material (e.g., metal, such as gold orplatinum) is formed on substrate 1000 to provide a working electrode 22for each sensor. The carbon or other electrode material can be depositedby a variety of methods including printing a carbon or metal ink, vapordeposition, and other methods. The printing may be done by screenprinting, gravure roll printing, transfer printing, and other knownprinting methods. The respective trace and contact pad 23 could beapplied together with working electrode 22, but may be applied in asubsequent step.

Similar to the working electrode 22, counter electrode 24 is formed onsubstrate 1000. The counter electrode(s) are formed by providing carbonor other conductive electrode material onto substrate 1000. In oneembodiment, the material used for the counter electrode(s) is a Ag/AgClink. The material of the counter electrode(s) may be deposited by avariety of methods including printing or vapor deposition. The printingmay be done by screen printing, gravure roll printing, transferprinting, and other known printing methods. The respective trace andcontact pad 25 could be applied together with counter electrodes 24, butmay be applied in a subsequent step.

Preferably, multiple sensors 10 are manufactured simultaneously; thatis, the working electrodes, including their traces and contact pads, fora plurality of sensors are produced (e.g., printed) on a polymer sheetor web, and simultaneously or subsequently, the counter electrodes, andtheir traces and contact pads, for a plurality of sensors are produced(e.g., printed). The working electrode(s) and counter electrode(s) canbe formed on separate substrates that are later positioned opposite oneanother so that the electrodes face each other. Alternately, to simplifyregistration of the substrates, the working electrodes can be formed ona first half of a substrate sheet of web and the counter electrodes areformed on a second half of the substrate sheet or web so that the sheetor web can be folded to superimpose the working and counter electrodesin a facing arrangement.

To provide sample chamber 20, spacer 15 is formed over at least one ofthe substrate/working electrode and substrate/counter electrode(s).Spacer 15 can be an adhesive spacer, such as a single layer of adhesiveor a double-sided adhesive tape (e.g., a polymer carrier film withadhesive disposed on opposing surfaces). Suitable spacer materialsinclude adhesives such as urethanes, acrylates, acrylics, latexes,rubbers and the like.

A channel, which will result in the sample chamber, is provided inspacer 15, either by cutting out a portion of the adhesive spacer orplacing two adhesive pieces in close proximity but having a gaptherebetween. The adhesive can be printed or otherwise disposed on thesubstrate according to a pattern which defines the channel region. Theadhesive spacer can be optionally provided with one or more releaseliners prior to its incorporation into the sensor. The adhesive can becut (e.g., die-cut or slit) to remove the portion of the adhesivecorresponding to the channel prior to disposing the spacer on thesubstrate.

Any sensing chemistry is disposed onto the substrate in at least thesample chamber regions. If any of the sensing chemistry component(s) isnon-leachable, that component is preferably disposed on the workingelectrode. If any of the sensing chemistry component(s) is diffusible,that component can be disposed on any surface of the substrate in thechannel region. The redox mediator and/or electrode transfer agent canbe disposed independently or together on the substrate prior to or afterplacement of the spacer. The redox mediator and/or electrode transferagent may be applied by a variety of methods including, for example,screen printing, ink jet printing, spraying, painting, striping along arow or column of aligned and/or adjacent electrodes, and the like. Othercomponents can be deposited separately or together with the redoxmediator and/or electrode transfer agent; these components can include,for example, surfactants, polymers, polymer films, preservatives,binders, buffers, and cross-linkers.

After disposing the spacer, redox mediator, second electron transferagent, sensing layers, and the like, the first and second substrates(having the working and counter electrodes thereon) are positionedopposite each other to form the sensor. The faces of the substrate arejoined by the adhesive of the spacer. After bringing the faces together,individual sensors can be cut out from the web of sensors using avariety of methods including, for example, die cutting, slitting, orotherwise cutting away the excess substrate material and separating theindividual sensors. In some embodiments, a combination of cutting orslitting methods is used. As another alternative, the individual sensorcomponents can first be cut out of the substrates and then broughttogether to form the sensor by adhesively joining the two components,such as by using the spacer adhesive.

The sides of the sensor can be straight to allow the sensor to be cutout from the remainder of the substrate and/or from other sensors byslitting the substrate in parallel directions using, for example, a gangarbor blade system. The edges of the sensor can define edges of thesample chamber and/or measurement zone. By accurately controlling thedistance between cuts, variability in sample chamber volume can often bereduced. In some instances, these cuts are parallel to each other, asparallel cuts are typically the easiest to reproduce.

Application of the Sensor

A common use for the analyte sensor of the present invention, such assensor strip 10, 10′, 100 is for the determination of analyteconcentration in a biological fluid, such as glucose concentration inblood, interstitial fluid, and the like, in a patient or other user.Sensor strips 10, 10′, 100 may be available at pharmacies, hospitals,clinics, from doctors, and other sources of medical devices. Multiplesensor strips 10, 10′, 100 may be packaged together and sold as a singleunit; e.g., a package of 25, 50, or 100 strips.

Sensor strips 10, 10′, 100 can be used for an electrochemical assay, or,for a photometric test. Sensor strips 10, 10′, 100 are generallyconfigured for use with an electrical meter, which may be connectable tovarious electronics. A meter may be available at generally the samelocations as sensor strips 10, 10′, 100 and sometimes may be packagedtogether with sensor strips 10, 10′, 100, e.g., as a kit.

Examples of suitable electronics connectable to the meter include a dataprocessing terminal, such as a personal computer (PC), a portablecomputer such as a to laptop or a handheld device (e.g., personaldigital assistants (PDAs)), and the like. The electronics are configuredfor data communication with the receiver via a wired or a wirelessconnection. Additionally, the electronics may further be connected to adata network (not shown) for storing, retrieving and updating datacorresponding to the detected glucose level of the user.

The various devices connected to the meter may wirelessly communicatewith a server device, e.g., using a common standard such as 802.11 orBluetooth RF protocol, or an IrDA infrared protocol. The server devicecould be another portable device, such as a Personal Digital Assistant(PDA) or notebook computer, or a larger device such as a desktopcomputer, appliance, etc. In some embodiments, the server device doeshave a display, such as a liquid crystal display (LCD), as well as aninput device, such as buttons, a keyboard, mouse or touch-screen. Withsuch an arrangement, the user can control the meter indirectly byinteracting with the user interface(s) of the server device, which inturn interacts with the meter across a wireless link.

The server device can also communicate with another device, such as forsending glucose data from the meter and/or the service device to a datastorage or computer. For example, the service device could send and/orreceive instructions (e.g., an insulin pump protocol) from a health careprovider computer. Examples of such communications include a PDAsynching data with a personal computer (PC), a mobile phonecommunicating over a cellular network with a computer at the other end,or a household appliance communicating with a computer system at aphysician's office.

A lancing device or other mechanism to obtain a sample of biologicalfluid, e.g., blood, from the patient or user may also be available atgenerally the same locations as sensor strips 10 and the meter, andsometimes may be packaged together with sensor strips 10 and/or meter,e.g., as a kit.

Integrated Sample Acquisition and Analyte Measurement Device

An analyte measurement device constructed according to the principles ofthe present invention typically includes a sensor strip 10, 10′, 100, asdescribed hereinabove, combined with a sample acquisition apparatus toprovide an integrated sampling and measurement device. The sampleacquisition apparatus typically includes, for example, a skin piercingmember, such as a lancet, that can be injected into a patient's skin tocause blood flow. The integrated sample acquisition and analytemeasurement device can comprise a lancing instrument that holds a lancetand sensor strip 10, 10′, 100. The lancing instrument might requireactive cocking. By requiring the user to cock the device prior to use,the risk of inadvertently triggering the lancet is minimized. Thelancing instrument could also permit the user to adjust the depth ofpenetration of the lancet into the skin. Such devices are commerciallyavailable from companies such as Boehringer Mannheim and Palco. Thisfeature allows users to adjust the lancing device for differences inskin thickness, skin durability, and pain sensitivity across differentsites on the body and across different users.

In one embodiment, the lancing instrument and the meter are integratedinto a single device. To operate the device the user need only insert adisposable cartridge containing a sensor strip and lancing device intothe integrated device, cock the lancing instrument, press it against theskin to activate it, and read the result of the measurement. Such anintegrated lancing instrument and test reader simplifies the testingprocedure for the user and minimizes the handling of body fluids.

In some embodiments, sensor strips 10, 10′ may be integrated with both ameter and a lancing device. Having multiple elements together in onedevice reduces the number of devices needed to obtain an analyte leveland facilitates the sampling process.

For example, embodiments may include a housing that includes one or moreof the subject strips, a skin piercing element and a processor fordetermining the concentration of an analyte in a sample applied to thestrip. A plurality of strips 10, 10′, 100 may be retained in a cassettein the housing interior and, upon actuation by a user, a single strip10, 10′ may be dispensed from the cassette so that at least a portionextends out of the housing for use.

Operation of the Sensor Strip

In use, a sample of biological fluid is provided into the sample chamberof the sensor, where the level of analyte is determined. The analysismay be based on providing an electrochemical assay or a photometricassay. In many embodiments, it is the level of glucose in blood that isdetermined. Also in many embodiments, the source of the biological fluidis a drop of blood drawn from a patient, e.g., after piercing thepatient's skin with a lancing device, which could be present in anintegrated device, together with the sensor strip.

The analyte in the sample is, e.g., electrooxidized or electroreduced,at working electrode 22, and the level of current obtained at counterelectrode 24 is correlated as analyte concentration.

Sensor strip 10, 10′, 100 may be operated with or without applying apotential to electrodes 22, 24. In one embodiment, the electrochemicalreaction occurs spontaneously and a potential need not be appliedbetween working electrode 22 and counter electrode 24. In anotherembodiment, a potential is applied between working electrode 22 andcounter electrode 24.

The invention has been described with reference to various specific andpreferred embodiments and techniques. However, it will be apparent toone of ordinarily skill in the art that many variations andmodifications may be made while remaining within the spirit and scope ofthe invention.

All patents and other references in this specification are indicative ofthe level of ordinary skill in the art to which this invention pertains.All patents are herein incorporated by reference to the same extent asif each individual patent was specifically and individually incorporatedby reference.

1.-19. (canceled)
 20. An electrochemical sensor strip for determining aconcentration of ketone bodies in blood or interstitial fluid, thesensor comprising: a first substrate having a first major surface and asecond major surface opposing the first major surface, the firstsubstrate defining a proximal end of the strip, a distal end of thestrip and first and second side edges of the strip extending from theproximal end to the distal end; a second substrate having a first majorsurface and a second major surface opposing the first major surface, thefirst and second substrates being disposed so that the first majorsurface of the first substrate is in facing relationship with the firstmajor surface of the second substrate; spacer material disposed betweenthe first substrate and the second substrate, the spacer material, thefirst substrate and the second substrate further defining: a firstaperture along the proximal end and between the substrates, a secondaperture along the first side edge and between the substrates, a channelleading from the first aperture to the second aperture, and a samplechamber adjacent the first aperture and along the channel, the samplechamber comprising a measurement zone having a volume of no more than 1μL; a working electrode disposed on the first major surface of the firstsubstrate; a ketone body responsive enzyme disposed on the workingelectrode; a counter electrode disposed on the first major surface ofone of the first substrate and the second substrate, the workingelectrode and the counter electrode being positioned relative to thesample chamber to generate a ketone body responsive signal when samplecontaining ketone bodies is disposed in the measurement zone; and aninsertion monitor disposed on one of the first and the second majorsurfaces of one of the first substrate and the second substrate, theinsertion monitor including a conductive stripe extending across a widthof the sensor strip and the insertion monitor providing a path forelectrical current between at least two contact leads of a meter. 21.The sensor according to claim 20, wherein the insertion monitor isdisposed on the second major surface of the first substrate.
 22. Thesensor according to claim 20, wherein the counter electrode is disposedon the first major surface of the second substrate.
 23. The sensoraccording to claim 20, wherein the insertion monitor has two or morecontact regions for electrical contact with the meter.
 24. The sensoraccording to claim 20, wherein the insertion monitor is configured andarranged to provide encoded information about the strip.
 25. Anelectrochemical sensor strip for determining the level of ketone bodiesin a sample of blood or interstitial fluid, the sensor strip having afirst side edge and a second side edge, the sensor strip comprising: afirst substrate having a first major surface and a second major surfaceopposing the first major surface, the first substrate defining aproximal end of the strip, a distal end of the strip and first andsecond side edges of the strip extending from the proximal end to thedistal end; a second substrate having a first major surface and a secondmajor surface opposing the first major surface, the first and secondsubstrates being disposed so that the first major surface of the firstsubstrate is in facing relationship with the first major surface of thesecond substrate; spacer material disposed between the first substrateand the second substrate, the spacer material, the first substrate andthe second substrate further defining: a first aperture along theproximal end and between the substrates, a second aperture along thefirst side edge and between the substrates, a channel leading from thefirst aperture to the second aperture, and a sample chamber adjacent thefirst aperture, the sample chamber comprising a measurement zone havinga volume of no more than 1 μL; a working electrode on the first majorsurface of the first substrate; a ketone body responsive enzyme disposedon the working electrode; a counter electrode on the first major surfaceof one of the first substrate and the second substrate, the workingelectrode and the counter electrode being positioned relative to thesample chamber to generate a ketone body responsive signal when samplecontaining ketone bodies is disposed in the measurement zone; and aninsertion monitor comprising electrically conductive material forming astripe extending from the first side edge to the second side edge of thesensor strip on one of the substrate surfaces.
 26. The sensor strip ofclaim 25, comprising two working electrodes.
 27. The sensor strip ofclaim 5, wherein the counter electrode is on the first major surface ofthe second substrate.